No matter how well built a plastic injection mold is, eventually it is going to require some kind of mold repair. Probably the most common method is to use laser welding or micro welding.
Both of these types of welding are truly amazing, for their flexibility and precision. Some of the welds I have seen over the years almost defy physics. A good welder is able to place a tiny dot of weld in exactly the right spot, even if it is in-between other critical areas and down inside a rib area. Think of a dot the size of a poppy seed, place exactly on the tip of a needle, all without disturbing any other details nearby!
This type of mold repair has saved the day countless times. When a molding machine is running at full production and a component breaks or wears out, time is of the essence. This is where the laser or micro welder really shines.
Once the broken or worn area is built up, it must be re-machined to bring it back to it's original shape. This can be done by hand, using diamond files and aluminum oxide abrasive stones. Or, it can be reworked by hand using an ultra-sonic polisher, which uses diamond or ceramic stones to work the steel.
Very often, the EDM machine is preferred, because the original shape was quite possibly made by EDM machining in the first place. The Charmilles-Agie EDM machines are very good at this type of work, as is the Makino and Sodick EDM.
It is possible that the original copper or Poco graphite electrodes are in storage, and can be reused. With the System 3R and Erowa precision tooling, it is an easy matter to save the electrodes for later use. If not, it is quickly remade in the high speed milling machine or surface grinder.
Another option is to hard machine the component in a CNC milling machine. By using coated carbide end mills or inserts, it is quite easy to re-machine the weld down to the original steel shape. There are micro ball end mills that work quite well for this process. Popular carbide cutters are manufactured by Sandvik, Iscar, Ingersoll and Valenite.
Once the shape is restored, it must be hand polished to give the specified surface finish. Mold polishing is somewhat of a specialty and takes years to truly master. The same aluminum oxide and ceramic stones are used as before, but in a finer grit to obtain a better finish.
The use of synthetic diamond polishing compound will likely be required to bring the steel to a nice lustre or mirror finish. A very popular company, Gesswein, makes or distributes everything related to mold polishing.
Now, the mold must be re-assembled and put back into production. Usually some tests and inspection reports must be made to verify that everything is in order and up to specification.
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